Many applications require a predetermined DC or very low frequency resistive value be synthesized from another given resistance or impedance value.
The need for this is felt especially in telephone applications and speech circuits.
Such circuits are, in fact, required to have different resistances or impedances, depending on whether the signal to be processed is a DC or AC signal.
In this respect, each telephone company administration sets the values for these two different impedances, and the values change with the companies. For instance, for DC resistance, values in the 20 to 100 Ohms are generally provided by most telephone circuits, whilst values in the 200 to 1400 Ohms are provided for AC resistance.
Further, a frequent requirement is that the AC impedance be a complex type.
Heretofore, these two different impedances have been implemented by a complex structure external to the telephone integrated circuit using, moreover, comparatively expensive precision components. One prior approach uses a capacitor outside the telephone integrated circuit. This capacitor is DC operated in an open loop, and under these conditions, it has a low impedance value across it, set to approximately 30 Ohms.
Beyond a certain frequency, the capacitor becomes the equivalent of a short by completing a control and amplification loop internal of the integrated circuit. The completion of the control loop basically allows the previous impedance value to be stepped up to 600 Ohm.
While achieving its objective, this prior approach has a drawback in that, since the capacitor should be able to short out already at a low frequency, its value must be quite high, thereby making the integration of said capacitor to the circuit either inconvenient or impossible.
This prior system requires, however, pins be added to the integrated circuit to which the capacitor is connected, resulting in higher cost as well as lower reliability.
A second, more complicated prior approach is described in an article "A software programmable CMOS telephone circuit", IEEE Journal of Solid-State Circuits, Vol. No. 7, July, 1991, wherein reference is made to a telephone circuit equipped with no less than six external components.
This second approach provides for two different impedances, DC and AC, to be implemented by two discrete control loops. The two signal components are separated by two external decoupling capacitors.
But even this prior approach is beset with the same drawbacks previously considered.